1. Field of the Invention
The present invention relates to a crane equipped with a unit having a plurality of wheels and capable of traveling on the ground in a turning direction.
2. Description of the Background Art
Heretofore, there has been known a crane equipped with a travelable counterweight unit, as described, for example, in Patent Documents: JP 2895434B, JP 2895437B and JP 02-005665B. This counterweight unit is suspended, for example, from a mast of the crane. In this state, accompanying a slewing movement of an upper slewing body of the crane, the counterweight unit can travel on the ground with respect to a direction of the slewing movement. Meanwhile, in a situation where the crane is operated to lift up a suspended load of a predetermined mass or more, the counterweight unit is floated up from the ground.
In many cases, the above type of counterweight unit is connected to the upper slewing body of the crane, through a connection member. In one example shown in FIG. 9B, the counterweight unit 90 is connected through a connection member 92 to an upper slewing body 91 of a crane, specifically, joined to the connection member 92 through a pin so as to be inclinable with respect to the ground. In another example shown in FIG. 9C, the counterweight unit 90 is connected through two connection members 92 to the upper slewing body 91 at respective two upper and lower positions, thus being restrained so as to be precluded from inclination to the ground. Alternatively, there can be some cases where the counterweight unit 90 is not directly connected to the upper slewing body 91.
Any of the above counterweight units is provided with a plurality of wheels, whose orientation is set to a direction aligned with a tangent line to a turning direction of the wheel; however, in fact, a turning radius of the wheel of the counterweight unit is increased, which is likely to cause various disadvantages. Specifically, as shown in FIG. 9A, in the conventional counterweight unit, although the orientation of each of the wheels 94 (a front-rear direction of the wheel) is adjusted to make agreement with a tangent line L1 to an orbit C of the wheel 94, an actual position of the wheel 94 is deviated outwardly from the normal orbit C, i.e., a circular trajectory having a radius r, due to a centrifugal force acting on the counterweight unit being traveling in a turning direction, involving an inadequate increase in a turning radius of the wheel 94. The increase in the turning radius causes the following disadvantages.
Firstly, in the case of the counterweight unit not directly connected to the upper slewing body, the counterweight unit is normally located immediately below a distal end of a mast; however, the increase in the turning radius of the wheel increases a turning radius of the entire counterweight unit, thus displacing the counterweight unit toward a rearward side with respect to the upper slewing body from the position under the distal end of the mast. If, in this state, the crane lifts up a suspended load of a predetermined mass or more to thereby float the counterweight unit from the ground, i.e., release the restraint of the counterweight unit by friction with the ground, the counterweight unit is returned to a position immediately below the distal end of the mast, i.e., a position corresponding to the normal turning radius r, by gravity acting on the counterweight unit. This makes the counterweight unit swing in a direction of the turning radius.
Secondly, in the case of the counterweight unit 90 connected through the connection member 92 to the upper slewing body 91, as shown in FIGS. 9B and 9C, the increase in the turning radius of the wheel 94 cannot vary the turning radius r of a unit body of the counterweight unit 90; however, this case also involves the following disadvantage. In the case of the inclinable counterweight unit 90 as shown in FIG. 9B, the increase in turning radius r of the wheel 94 does not increase the turning radius r of the unit body of the counterweight unit 90, which gives a difference between the turning radius of the unit body of the counterweight unit 90 and the turning radius of the wheel 94, the difference undesirably inclining the entire counterweight unit 90. Particularly, in the case where the wheel 94 is easily deformable such as a pneumatic tire, i.e., a tire to be used in air-filled state, the inclination of the entire counterweight unit 90 is more pronounced. The inclination of the counterweight unit 90 makes respective loads applied to the wheels 94 be ununiform, thereby accelerating wear of the wheel 94 and shortening wheel life. On the other hand, in the case where the counterweight unit 90 is not inclinable as shown in FIG. 9C, the increase in the turning radius of the wheel 94 causes the wheel 94 to undergo a significant shear force to thereby bring the wheel 94 into abnormal deformation. Hence, in this case, damage and wear of the wheel 94 is accelerated, resulting in shortened wheel life.
As means to avoid the above disadvantages, it is conceivable to perform an operation of returning the turning radius of the wheel to an adequate value, for example, an operation of changing the orientation of the wheel from the state shown in FIG. 9A, linearly moving the counterweight unit 90 to an inward side of the orbit C, and thereafter returning the orientation of the wheel to the state shown in FIG. 9A again; however, this operation is complicated and time-consuming, causing deterioration in efficiency of crane operations.